Adsorption dryers for compressed air and gases have been known for many years and are widely used throughout the world. Although other types of dryer are available, such as deliquescent and refrigeration dryers, these cannot give a pressure dew-point as low as that achieved by adsorption dryers and which is essential for many applications.
Normally, adsorption dryers are dual tower dryers. That is to say, they include two towers of desiccant material (commonly known as beds) one of which is ‘on stream’ drying the gas whilst the other is being regenerated. In a dual tower dryer, the gas to be dried is passed through the desiccant bed of the on-stream tower continuously, in one direction, during a drying cycle. Then, after a predetermined time interval (this interval being chosen such that the bed will have adsorbed sufficient moisture) the inlet gas is switched to the desiccant bed of the other tower and the first desiccant bed is regenerated by some suitable procedure such as heating, evacuation or passing a purge gas through it, usually in a flow direction opposite to the flow of gas to be dried.
Adsorption dryers are available in at least two distinct types: heat regenerative and heatless. A heat regenerative dryer, as the name implies, uses heat in one form or another to reactivate the wet desiccant bed normally in conjunction with a flow of purge gas. A heatless dryer uses a purge flow of dry gas, which is usually a proportion of dried gas from the on-stream tower, the purge gas being passed through the regenerating bed at a lower pressure than the gas in the on-stream tower. Both types of dryer are normally operated on a fixed time cycle for drying and regeneration and both cycles are usually of an equal duration, or they can be operated in a variable cycle. The cycle times for heat regenerative dryers are usually measured in hours whereas for heatless dryers they are measured in minutes.
To control the flow of gas from one tower to the other, and to control the purge gas, a series of valves is employed. These valves most typically include inlet valves which switch the gas from one tower to the other, exhaust valves which control the duration of purge gas flow and repressurisation of the towers, and outlet check valves which prevent the outlet stream pressurizing the off-stream bed. In addition to these valves, a number of other valves such as purge check valves, repressurisation valves, additional exhaust restrictor valves and so forth may be required.